Dopaminergic neurotransmission may be involved in learning, reinforcement of behaviour, attention, and sensorimotor integration(1,2). Binding of the radioligand C-11-labelled raclopride to dopamine D-2 receptors is sensitive to levels of endogenous dopamine, which can be released by pharmacological challenge(3-8). Here we use C-11-labelled raclopride and positron emission tomography scans to provide evidence that endogenous dopamine is released in the human striatum during a goal-directed motor task, namely a video game. Binding of raclopride to dopamine receptors in the striatum was significantly reduced during the video game compared with baseline levels of binding, consistent with increased release and binding of dopamine to its receptors. The reduction in binding of raclopride in the striatum positively correlated with the performance level during the task and was greatest in the ventral striatum. These results show, to our knowledge for the first time, behavioural conditions under which dopamine is released in humans, and illustrate the ability of positron emission tomography to detect neurotransmitter fluxes in vivo during manipulations of behaviour.